RE: Best Way to charge gel 7a 12v on way to field?
How much current a 12v battery will take during a charge relates to the charge voltage as well as the discharge state of the battery. A fully charged lead-acid battery (either gel-cell, wet-cell, AGM, etc) will exhibit a fairly high internal resistance when attempting to charge.
Any voltage less than 12v can be considered fully discharged, the resting voltage of a battery will be about 12.65 volts when fully charged. The resting voltage is measured after disconnecting from any charger or load, and left sitting for 24 hours. If you measured 13.4v on it, you had just recently disconnected it from a charger and it was still showing a surface charge.
A gel-cell battery that is in a discharged state can be safely charged at 14.4 volts and will take a reasonably high current (probably a couple of amps for a 7ah battery) without a problem. The problem is that once a fully charged state is reached, 14.4v is too high and will start to boil the electrolyte, causing pressure to build inside the battery. This can happen even if the charge current is less than 100ma. 13.5v would be a more reasonable charge voltage for long-term, but will not be enough voltage to do much charging in a one hour period.
What the best solution would be is to use an intelligent multi-stage charger, but you would probably only find one that operates from 120v AC, and you are back to using an inverter.
If it were me, here is what I would do. I would rig up a 12v cigarette lighter plug to a pigtail like you suggested, but instead of a fuse, I would use an automotive light bulb, say an 1141, that would limit the current to less than 1 amp. Even if you directly short the line, the worst that would happen is the 12v bulb will light up, and you would be drawing about 1.4 amps. Just make sure you put the bulb in the positive side of the pigtail! Most vehicles may start out at 14.4 v when started, but usually taper down the charge voltage some after running for a while. Even if the voltage stays above 14v, for one hour it should be fine, and the 12v bulb will have some voltage drop across it even when cold.
The reason the 12v bulb works in this case is that the resistance is not linear. The resistance is fairly low when the filament is cold, the resistance will increase as the filament warms up. The old Kraft battery chargers used bulbs to limit the current as well as indicate charging. They sold different bulbs for different charge rates.
Bob R.